K1 is valid usually for 6 months from date of issue or as specified in the "Valid until field". Note there is no requirement that you must go to the USA within 3 months or 90 days, only requirement is once you are in the USA you must get married within 90 days and hopefully until then your K-1 is valid too. If not, you can just file for AOS and the noa you receive in lieu of AOS protects your status.
Here is what you should do my friend:
Book your hotel and car rental through a US company (such as expedia, orbitz, hotwire). This way, you will not only get excellent prices but also your credit card will just be charged in US$! hence completely eliminating the need for lovely credit card fees.
It is extremely unwise to use a US credit or debit card in any foreign country for the below reasons:
1) Crappy exchange rates
2) Foreign transaction fees which can be upto 4% of EACH transaction
3) High chance of identity theft and bogus charges if you use them at shady places as they know these cards are from US
For everyday expenses, I recommend getting a Travelex debit card (its available from all major US airports). You put a certain amount of money in it and bam your exchange rate is locked, there are no transaction fees and there is no chance of identity theft. Plus if there is any unused money left when you return, you can get your US dollars at the same rate as you used to purchase the UK pounds!
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